The present invention relates to a gas discharge display panel, to a display apparatus including the panel and to methods of priming or scanning such panels.
A simple form of a gas discharge display panel comprises a two dimensional matrix of light-emitting elements such as glow discharge cells. The elements are connected between conductors at respective cross-points formed by two groups of coordinate conductors. Each of these elements can be illuminated selectively by simultaneously applying suitable energizing signals to the two conductors, one in each group, between which the element is connected, by an addressing circuit arrangement of a display apparatus.
In the interests of clarity, the words "row" and "column" will be used to distinguish between the coordinate lines of light emitting elements which form the two-dimensional matrix of a gas discharge display. The coordinate lines may extend at any desired angle, for example 90.degree., to each other. Thus either of the two groups of coordinate lines of elements can be termed "row" elements with the elements of the other group being termed "column" elements. The two groups of coordinate conductors which form the cross-points will be referred to, correspondingly, as "row" conductors or electrodes and "column" conductors or electrodes.
When using gas discharge display panels for displaying alphanumeric characters it is important that the cells break-down and luminesce at the desired time, otherwise the displayed information will be incorrect. With a simple type of panel it has been found that reliable breakdown of the cells cannot be ensured. Consequently refinements have been evolved to overcome this problem.
In order to appreciate these refinements it is necessary to understand the operation of a panel and the cells thereof.
For a satisfactory display using a recurrent scanning cycle mode of operation a field rate of at least 50 Hz is desirable in order to prevent flicker, that is, the addressed cells are pulsed 50 times per second. For each field scan, the actual period of energization of a cell depends on factors such as the number of cells on a panel and the way that they are pulsed or scanned. Thus, for a 200.times.200 element matrix scanned row-by-row, a row rate of 50.times.200=10 KHz is necessary. This means that the row dwell time is 100 .mu.S during which each element which is to be energized in a row should be held energized for as long a time as possible, during the 100 .mu.S in order to achieve maximum brightness. However, in the case of a glow discharge cell, at least 10 .mu.S of the row dwell time is taken up by an inherent delay which occurs before the discharge of an energised cell will ignite; and of the remaining 90 .mu.S during which the cell could be held energised, some of this 90 .mu.S is required for filling a column register in dependence on the coded electrical signals for the selective addressing of the cell columns. In order to keep the column addressing time at a maximum, the column register fill time may be 10 .mu.S, for example so that the actual column addressing time is 90 .mu.S. This means that the "on time" of the cells is 80 .mu.S due to their inherent delay.
This inherent delay is the result of two factors: a statistical lag controlled by the time that elapses before suitable initiatory ionization is produced in the cell by agencies internal or external to the panel, and a formative delay controlled by the gas discharge processes that must occur before weak but sufficient initiating ionization is amplified sufficiently to produce breakdown and formation of the discharge.
The formative delay is controlled by the nature of the gas, the electrode geometry and the voltage that is supplied to the cell. It can also be affected by the level of the initiating ionization in the cell. Normally delays caused by formative lag can be arranged not to be a problem for cyclic panel operation. However, statistical delays can be long, seriously affecting panel operation. The problem becomes more serious as the number n of row electrodes being cycled increases because all n electrodes must be scanned, i.e. pulsed, in less than 0.02/n sec. The total lag can be a significant fraction of this value and the cells will have variable discharge duration which can seriously affect the display appearance and brightness.
One refinement to a simple panel for improving the reliability of cell-breakdown and reducing the effect of statistical lag is to arrange for a small amount of ionization to be present in each cell either all the time the display system is being operated or just before the cell is to be broken down and a discharge established. If the ionization level is increased further, the formative lag can be reduced. In the case of the simple cyclic panel, the production of this small amount of ionization to each cell, which is referred to as "priming" the cell, is achieved in a variety of ways. The panel can be designed to have "keep-alive" cells, that is cells which pass a discharge for the whole time the panel is being operated, located around the perimeter of the display. Alternatively, these perimeter cells can be switched on once per cycle as part of the cyclic addressing system. These methods give a "picture-frame" effect that can be visible to the viewer or obscured by suitable opaque barriers, either internal or external to the panel. These methods become less effective as panel size increases because the distance from perimeter to the cells in the center of the panel increases.
In some commercially available panels, discharges are formed in cells which are not display cells but which are cells auxiliary to the display. These can be referred to as "priming or scanning cells" and can be located either behind the display cells and communicating with the display cells via small holes in the cathode common to both cells as disclosed in British Pat. Specification No. 1,317,221, and U.S. Pat. No. 3,766,420, or to one side of the display cells and in the same plane as the display cells, communicating with the display cells via apertures in the cell wall structure as disclosed in British Pat. Specification No. 1,481,941. These auxiliary cells are scanned in sequence along the cathode or column electrodes in the order first cathode, second cathode . . . last cathode and then reset to commence at the first cathode again. These priming discharges may or may not be visible to the viewer as a background glow affecting the contrast of the information being displayed.
The cathode-to-cathode scanning technique used gives rise to a limitation on the maximum number of columns of cells which can be provided in a single panel if flicker effects are to be avoided. For a field scan frequency of 50 Hz and a cathode dwell time of 100 .mu.S, the theoretical maximum number of columns of cells is 200.
This limitation is of particular importance in practical applications such as word processing, that is typing where the characters being typed are being stored on for example a floppy magnetic disc, magnetic tape or paper tape to be read by a computer, where the typist wants a temporary record of what has just been typed. For this purpose the display panel must be able to display at least 4 lines of 80 characters, both upper and lower case. For this purpose 480 columns of 48 cells are necessary, or 560 columns in the case of 2 blank spaces between characters.
U.S. Pat. No. 3,942,060 discloses a double layer panel which is divided internally into two portions, each portion having 200 columns of cells and its own display anode and cathode electrodes. The scanning electrodes of each portion are energized by respective drivers. Such a panel is structurally complicated.